1. Field of the Invention
This invention relates to resin dispersions, and particularly to an aqueous dispersion of olefin resins and a continuous process for the preparation thereof.
2. Description of the Prior Art
Known processes for preparing aqueous dispersions of olefin resins include processes using suspension polymerization or emulsion polymerization, e.g., a process including formation of an aqueous dispersion of ethylenevinyl acetate copolymer, and an after-dispersion process, which comprises dispersing a polymer formed by polymerization in water after carrying out the polymerization.
Aqueous dispersions prepared by suspension polymerization or emulsion polymerization are limited to polymers produced by radical polymerization. Furthermore, in the case of polymers having 70% by weight or more olefin content, the pressure during polymerization becomes remarkably high, namely, serveral hundred to several thousand Kg/cm.sup.2, because of the reactivity of the monomers. Therefore, from the industrial stand point aqueous dispersions of olefin resins obtained by this process have been limited to ethylene-vinyl acetate copolymer having, at most, 40% by weight or less ethylene content as described in, for example, Kobunshikako, page 5, July 18, 1977 (Japan).
On the contrary, since the after-dispersion process is the process which comprises dispersing a polymer obtained by a polymerization reaction in water after carrying out the polymerization reaction, it is not limited by the polymerization process, and it is possible to obtain aqueous dispersions of various polymers so as to have an olefin content from a low value to a high value.
The after-dispersion process includes various particular processes. Typical after-dispersion processes include a process which comprises dispersing a polymer in a fused state in hot water containing an emulsifying agent by means of a high-shear stirrer, such as a homomixer, etc. which is used for preparing an aqueous dispersion of polyethylene wax having a low molecular weight as described in, for example, Japanese Patent Application (OPI) No. 44146/76, and a process which comprises dissolving a polymer in an organic solvent, mixing the resulting solution with water containing an emulsifying agent, emulsifying by means of a high-shear stirrer, such as a kneader or a homogenizer, and evaporating the solvent from the resulted emulsion to obtain an aqueous dispersion of the polymer. The former process is not a process capable of general use, because it is difficult to apply to polymers having a high molecular weight and a large viscosity. Namely, in order to uniformly disperse the fused polymer in hot water by means of a high-shear stirrer, it is necessary to reduce the viscosity as low as possible. In case of resins having a high molecular weight and a high melt viscosity, however, the temperature to which the resin must be heated is very high and it is necessary for the whole system to have a high temperature and a high pressure. Consequently, the apparatus becomes large in scale, and the process is not suitable for practical use. In the latter process, also using the organic solvent, a step for removing the solvent after dispersion is required, and removal of the solvent while preventing foaming is technically difficult.
As a modified process of the latter process using the organic solvent, it has been proposed to continuously feed a resin to a hopper of a monoaxial screw extruder while continuously feeding an organic solvent and water containing an emulsifying agent to a feed inlet provided in a compression zone of the extruder, heating and kneading the resin in the extruder, and subsequently extruding while blending with the organic solvent and water in the amount at least 17% by weight which contains an emulsifying agent, as described in U.S. Pat. No. 3,356,629. However, in this process, the dispersion of the resin is insufficient, and a homogenous dispersion can not be obtained if the dispersing is not carried out by a high-speed stirrer together with the further addition of a large amount of water to the extruded product. Further, this process is also defective in that a step for removing the organic solvent is required, because use of a solvent is essential and a large amount of the solvent is used.